CHAPTER 6 THE THIRD LAW AND CRYOGENICS
6.3 CRYOGENICS 161
Sincemmagat constantBdecreases with increasingT, as explained above, we see that the
entropy change during isothermal magnetization isnegative.
By repeatedly carrying out a procedure of isothermal magnetization and adiabatic de-
magnetization, starting each stage at the temperature produced by the previous stage, it has
been possible to attain a temperature as low as0:0015K. The temperature can be reduced
still further, down to 16 microkelvins, by using adiabatic nuclear demagnetization. How-
ever, as is evident from the figure, if in accordance with the third law both of the entropy
curves come together at the absolute zero of the kelvin scale, then it is not possible to attain
a temperature of zero kelvins in a finite number of stages of adiabatic demagnetization. This
conclusion is called theprinciple of the unattainability of absolute zero.